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John McCormack

UNUSUAL VIBRATION

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My 1970 Pi has developed a vibration through the body from the back end. It only occurs on left hand bends, when the suspension compresses on undulations in the road and when accelerating hard at speed.

 

Background

 

About 2 months ago I fitted CV axles. There might have been a vibration then but I didn't get it out enough to really know. The front right diff mount was broken away and I probably assumed any problems were associated with that.

 

Last week I had the diff mounts repaired and strengthened by a Triumph specialist garage, He also put a new pinion seal in as the old one had a slight weep.

 

The car had rear telescopic shocks twhich were attached to the body on the wheel arch. I bought new mounts and shocks from the Triumph man and when I got it home I fitted them. I drove the car on a few 10 km local runs and was happy with it in suburban driving.

 

Yesterday I took her on a weekend TR Register run to test out the changes.

 

On the freeway at 110-120 km/h the vibrations from the rear end were concerning. Again, they only occur on left hand bends, when the suspension compressed on undulations in the road and when accelerating hard at speed. In a straight line and on right hand bends the car was vibration free at all speeds. There are no noises associated with the vibrations.

 

At a meeting with other cars 100kms from home I noticed a pool of oil under the car. A quick look and it was from the new pinion seal. While we were stopped I gave the rear end a bounce and it was obvious the new adjustable shocks were set too soft. As we were heading to a remote spot I decided to take the car home and join the others at our destination in another car.

 

On the run home I took it easy due to the oil leak, 70-90km/h. The vibration was no worse or better than before. Again, in a straight line or on right hand bends it was vibration free. Near home I gave her a burst in 2nd and there wasn't a noticeable problem.

 

I am assuming the leak and vibration were unrelated, there is no sign the pinion bearing has a problem and I am confident the garage would have picked any problem up when replacing the seal. I could be wrong.

 

My suspicions were that the vibration was caused by:

  • The body flexing enough on bends and when the suspension compressed to touch the new shock mounts. The new mounts are only a few mm clear of one now superfluous bolt that held the old body mount on. If the body flexed on corners, under acceleration or on undulations in the road the body might touch the shock mount transmitting road wheel vibration through the body. Only really noticeable at speed due to the increased vibration from the wheels.
  • The soft rear shocks were exacerbating the problem.
  • A bad diff pinion or axle bearing. I doubt this as a bearing problem would be there all the time, make a noise and get worse as time went on.
  • A defective CV axle. I don't know enough how these axles work to know whether a CV axle would induce vibration only in the situations they were occurring.

Any ideas thank you all? I will remove the offending bolts that are close to the new mounts but others may have experienced similar vibrations and it would be great to have other possibilities to look at while it is apart.

 

John

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Personally I’d ditch the rear type of rear shock mounts you have and either get the type that attach to where the original lever arms bolt to the chassis, or go back to the lever arms.**

I doubt if the diff is the problem as they normally get noisy first. Just in case it’s blocked and causing the leak check the ‘wiggle pin in the top of the diff is free to move (top left hand side) and vent any pressure that may build up when hot. The oil leak, best go back to who ever fitted the new seal.

Now the vibration,.... check the exhaust is not contacting anywhere. Hoofing it round a bend and it may move just enough to vibrate on the chassis. It is quite a tight fit especially through the ‘Tee Shirt’. So linked to that check the integrity of the engine and gearbox mounts.

Finally how are the wheel bearings on the rear?

** Type 2 of Type 3.

.

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Have you checked the distance between the body tub and the bump stop mount on the trailing arm? if you fit adjustable shocks with the type as above the shock will bottom out before the bump stop meets the tub therefore stressing the lever arm and ultimately failure of the trailing arm where the shock absorber is fixed to it. I fit a nominal 30mm spacer to stop this happening. As the shocks are set to soft this could be happening as the car turns that way the bump stop meets the tub before there is no travel left in the shock absorber. as it was designed to do. Clem

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Have you checked the distance between the body tub and the bump stop mount on the trailing arm? if you fit adjustable shocks with the type as above the shock will bottom out before the bump stop meets the tub therefore stressing the lever arm and ultimately failure of the trailing arm where the shock absorber is fixed to it. I fit a nominal 30mm spacer to stop this happening. As the shocks are set to soft this could be happening as the car turns that way the bump stop meets the tub before there is no travel left in the shock absorber. as it was designed to do. Clem

Thank you all for the advice. Just to clarify, the new telescopic shock mounts are bolted to the original lever arm mounts and the new shocks have internal bump stops. The axles included new upgraded hubs.

I removed the wheels and the rear end looks to be all in order, no tyre rubbing, no contact with shock mounts or exhaust, shock travel is right, no diff bearing issues.

After talking with a couple of mechanics including the one who provided the CV axles,and in the absence of any other cause, we have deduced it is the drivers side CV axle.

The axle was fitted a couple of months ago and I noticed a slight vibration on its only other very short run above 80km/h. This was its first longer run and at above 100Km/H.

When the axle isn't under load with the car running in a straight line on smooth roads the axle is fine with no noticeable vibration. Once the axle is loaded up on left hand curves, when the rear end compresses on undulations or under acceleration at speed one or more of the bearings in the CV mechanism is probably binding a bit causing the vibration. It is only noticeable at speeds above about 80 km/h and really a problem above 100 km/h.

I bought the shocks and CV axles from the same mechanic who did the diff mounts and pinion seal. The car is going back to him in two weeks and he will redo the pinion seal and put another axle on the drivers side.

Hopefully that will cure it. In the meantime the TR2 is in great working order and remains my daily driver.

The weather is cooling down down under but life could be worse.

Edited by John McCormack

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